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%I #7 Mar 04 2014 11:54:58
%S 4,10,20,25,27,29,30,31,30,31,30,31,30,31,30,31,30,31,30,31,30,31,30,
%T 31,30,31,30,31,30,31,30,31,30,31,30,31,30,31,30,31,30,31,30,31,30,31,
%U 30,31,30,31,30,31,30,31,30,31,30,31,30,31,30,31,30,31,30
%N a(n) = number of different positive integers that can be formed from different groupings of expressions of the form n op1 n op2 n op3 n, where each of op1, op2 and op3 are addition, subtraction, multiplication or division.
%C If one uses all 4^3=64 forms of this type but no parentheses, the sequence starts 4,9,15,13,15,14... In this case 4/4/4/4=1/4/4=1/16 is not an integer (association left-to-right), whereas with parenthesis one could write (4/4)/(4/4)=1, an integer, for example. The definition need clarification in this respect. [From _R. J. Mathar_, Jan 22 2009]
%F If k >3, a(2k-1)=30 and a(2k)=31. - _Ken Levasseur_, Oct 01 2008
%e You can form the numbers 1, 2, 3, 4 with 4 ones; hence the first term is 4.
%K easy,nonn
%O 1,1
%A _Ken Levasseur_, Sep 29 2008
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